CH 49 86 47 describes an apparatus for the absorption of e.g. SO3, which likewise includes two different absorption stages, wherein the one is designed as Venturi absorber and the other one as absorber with ebullient liquid, but without fixed-bed packing. In this configuration it is advantageous that it can be realized such that the sulfuric acid sump at the exit end of the Venturi coincides with the sulfuric acid sump at the bottom of the fixed-bed absorber and thus a common sump exists. The acid then flows in the same flow direction as the SO3 gas to be absorbed.
A similar process is also known from CH 54 72 31, which describes an intermediate absorber for the absorption of SO3. This intermediate absorber consists of a Venturi absorber and a downstream settling space and separating device for the sulfuric acid still contained in the gas.
CH 57 8 86 describes a process for the absorption of SO3 or moisture from gaseous media by means of sulfuric acid, wherein the largest part of the absorption takes place in a first absorption stage in a vertically arranged Venturi absorber in co-current flow between injected sulfuric acid and the gaseous medium. Above the sulfuric acid sump of the Venturi absorber, a connection to a second absorption stage is arranged. The second absorption stage is a vertically arranged tower provided with a packed bed, in which the absorption is effected in counter-current flow in that the gaseous medium is guided from bottom to top and the sulfuric acid is sprayed onto the packing from above and from there trickles down.
In the industrial production of sulfuric acid it is of considerable economic importance that all individual steps are distinctly exothermal, namely the oxidation (combustion) of the sulfur to SO2, the oxidation of SO2 to SO3 (catalytic conversion), the hydration of SO3 with water (H2O) to sulfuric acid (H2SO4), and its dilution to a technical concentration of e.g. 98.5% H2SO4. The energy released by these chemical reactions can be used for the production of high-pressure steam and/or low-pressure steam, which then can be utilized for generating electricity, for other process applications or for heating purposes.
Such energy recovery (also heat recovery) must be designed as efficient and comprehensive as possible, in order to convert an optimum of the available heat quantity into steam and thus maximize the economic profitability of a sulfuric acid plant. A large part of the energy (about 60-70%) is available on a sufficiently high temperature level and can be converted directly into high-pressure steam by means of suitable heat exchangers (waste heat boiler, economizer, superheater) in a known way. A smaller part of the energy (about 30-40%) is available on a lower temperature level and therefore can only be +++++ low-pressure steam, and this only incompletely. However, the latter heat recovery and the interconnection of the recovered energy within the plant lead to a considerably increased control and regulation effort. In addition, the heat exchangers for the recovery of heat from the sulfuric acid used for absorption possibly are exposed to a strong corrosion in the sulfuric acid production, namely when the concentration of the sulfuric acid is not optimally maintained (to 98.5-99.8% H2SO4) and due to the presence of excess water falls below this optimum concentration. This corrosion is increasingly dramatic the further the concentration lies below the optimum.
DE 10 2010 006 541 A1 likewise describes a process for the production of sulfuric acid with a particular focus on the cooling of the acid. When acid which is withdrawn from an absorption apparatus of a sulfuric acid plant, the acid is pumped from the acid pump tank into a heat exchanger for cooling and subsequently again supplied to the absorption apparatus, wherein in the heat exchanger the acid heats water as heat transport medium and at least partly converts it into steam. It is provided that the acid is guided in the tube side of the heat exchanger and the water is guided in the shell side and this water is at least partly converted to steam.
U.S. Pat. No. 4,996,038 describe a process and a plant for heat recovery during the production of sulfuric acid. Sulfur trioxide is absorbed in hot, concentrated sulfuric acid with a concentration between 98 and 101% and a temperature greater than 120° C. The absorption is effected in two stages, a primary and a secondary absorption, wherein both apparatuses are designed as packed bed tower and the SO3-containing gas each is guided in counter-current upward flow to the sulfuric acid which is fed to each packed bed from above. The collected hot acid from both absorption stages, which flows off at the bottom, then is supplied to a heat exchanger in which low-pressure steam is generated.
All processes have in common that in the case of a leakage in a heat exchanger within the heat recovery system, the plant generally must be shut down completely. In addition, the risk of water as coolant mixing with sulfuric acid as medium to be cooled involves considerable risks, so that a particularly fast interruption of the respective leakage is required.